Why is London the only major global sized city in the UK?

Other countries have multiple global sized “hub” cities. But in the UK everything seems to be centred primarily in London with a network of more provincial and smaller cities branching off of London. Why is this?

I dont think the UK is that different from any other small sized country. There is one major city, which in London's case dates to the stone age really due to the river, and then a smaller city in the other major regions. You cant compare the UK to a large country. But in general large cities form from the ability to trade.